Hollow spherical glass particles, also known in the state of the art as “synthetic glass microspheres” or “glass microbubbles” or “glass microballoons”, typically have low specific gravity, satisfactory heat resistance, heat insulating properties, pressure-resistance (e.g., crush strength) and impact resistance, and may achieve superior physical properties in comparison to conventional fillers. Each hollow spherical glass particle has an essentially spherical form and an essentially spherical inner void.
Due to their advantageous properties the hollow spherical glass microspheres are used in a variety of areas and applications. For example, the hollow spherical glass microspheres are used as light-weight fillers for composite polymeric materials of different kinds or in cryogenic technology, for fabrication of acoustic and thermal insulating materials or as targets for laser thermonuclear synthesis. An overview of the state of the art regarding the use, properties and technology of the hollow spherical glass particles can be found for example in “Hollow glass microspheres. Use, properties, and technology (Review)” by V. V. Budov in Science In Glass Production, Glass and Ceramics, July 1994, Volume 51, Issue 7, pp 230-235.
Several methods for producing hollow spherical glass particles have also been developed and are described in the prior art. Early methods for manufacturing hollow glass microspheres involved for example combining sodium silicate and boric acid with a suitable foaming agent, drying (for example in a spray dryer) or crushing the mixture with addition ingredients (for example in a ball mill with a suspension of water, china clay, feldspars, metakaolin, sodium silicate and/or potassium silicate, zeolites, sodium carbonate and/or potassium carbonate and/or calcium carbonate and/or magnesium carbonate, aluminium hydroxide etc.), adjusting the size of the crushed particles and drying the mixture in a spray dryer in order to achieve granules. Subsequently the granules are fired. The firing temperature achieves values of between about 1200° C. and 1800° C. However, these methods have a drawback that starting materials such as boric acid are required that can result in the formation of toxic compounds during production of and/or while using the hollow spherical glass particles.
U.S. Pat. No. 7,666,505 B2 describes hollow spherical glass particles comprising aluminosilicate and methods of making same. The hollow spherical glass microspheres described therein comprise 5.2 wt. % to 30 wt. % calcium oxide and greater than 4 wt. % to less than about 10 wt. % sodium oxide, wherein the microspheres have a total alkali metal oxide content of less than about 10 wt. %. In addition, U.S. Pat. No. 7,666,505 B2 describes that the presence of relatively high percentage of sodium oxide results in a poor chemical durability of the hollow spherical glass particles.
U.S. patent application Ser. No. 09/858,571 (Pub. No: US 2001/0043996 A1) and U.S. patent application Ser. No. 14/440,249 (Pub. No: US 2015/0315075 A1) describe hollow glass aluminosilicate microspheres and processes for their production. The mechanical durability of these microspheres is higher due to boron trioxide (B2O3). However, as described above, the presence of boron that may lead to toxic boron compounds is undesirable. Moreover, the presence of boron trioxide lowers the melting temperature of the microspheres.